1. Field of the Invention
This invention relates to a piston and cylinder arrangment for internal combustion engines, and more particularly for internal combustion engines designed to run on diesel and similar fuels.
2. Description of the Prior Art
Traditionally direct injection diesel engines that are naturally aspirated and run on heavier fuels, such as diesel, require high compressions ratios of at least 17:1, and preferably in the order of 22 to 1 for precombustion chamber engines, to operate successfully with relatively clean exhaust. This means that the engine must be made strong enough to withstand the cylinder pressures associated with these compression ratios, which in turn makes the engine heavy and cumbersome. Diesel engines have, however, certain advantages that make them suitable for applications where durability is important. The absence of an electrical ignition system makes them less prone to break down in moist environments, and their relative mechanical simplicity enhances their ruggedness. An important attractive feature of diesel engines is the lower volatility of diesel fuels, which permits the engines to be transported on aircraft or other vehicles where the high volatility of gasolines is problematic. The transportation of gasoline engines, for example for powering generator sets, is not permitted on military aircraft.
Unfortunately, the bulkiness of diesel engines has hitherto been a major disadvantage in mobile applications, especially when they have to be delivered to site by aircraft. The key to reducing the weight of the engine is to use lower compression ratios, ideally of the order of 14/15:1 or lower, as for conventional lightweight gasoline engines. Unfortunately, with these compressions ratios it is hard to get diesel fuels to ignite.
Previous attempts have been made to run an engine on diesel or kerosene-based fuels at low compression ratios using spark ignition systems. This has been recently demonstrated at South West Research Institute (SWRI) in a joint project (1) between SWRI and Sanshin Industries (a unit of Yamaha). In the SwRI/Sanshin (S/S) project, an energy cell (EC) approach was developed to improve combustion efficiency at the end of the combustion stroke. This made it possible to produce 21.3 kW (IHP) and 160 gm/hp/Hr Indicated Specific Fuel Consumption (ISFC) at 5500 rpm. The problem encountered with this system was fuel inefficiency at full and part loads. An efficient diesel engine will consume approximately 130 gm/hp/H, and the full benefit of diesel efficiency could not be realized due to heat transfer through the cylinder head via the energy cell and the low compression ratios of 10:1. Also, apart from detracting from one of the main advantages of the diesel engine, namely the absence of an electrical ignition system, such systems have not undergone much further development, as they tend to have a dirty, exhaust with a high levels of exhaust emission of mainly unburned hydrocarbons.